Differential amplifiers are commonly employed as key components in many integrated circuits. However, the usefulness of differential amplifiers in many instrumentation type applications is often limited by non-linearities in their gain characteristics. These non-linearities also limit the dynamic range of input signal voltage magnitudes over which the amplifiers can be employed.
The linear dynamic range of a differential amplifier can be extended through the use of "emitter-degeneration" as shown in FIG. 1. In accordance with this approach, the amplifier is provided with a pair of constant-current sources separately connected to the emitters of the transistors of the amplifier and further an "emitter feedback" resistor is connected between the emitters of these transistors. This type of circuitry reduces non-linearities in the gain of a differential amplifier but only to a limited extent.
To more fully resolve the problem and linearize the gain of a differential amplifier, circuit designers sometimes employ the "cascomp" circuit shown in FIG. 2. The voltage between the emitters of the cascode transistors Q.sub.3 and Q.sub.4 is nominally equal to the net base-to-emitter voltages of the primary differential transistors Q.sub.1 and Q.sub.2, which voltages vary non-linearly with the output signal and thereby cause the overall non-linearity of the amplifier. A compensating amplifier A.sub.cmp produces an amplified non-linear signal which is added to the output of the differential amplifier. The gain of the amplifier A.sub.cmp is adjusted so that its output should cancel the non-linear components of the output of the differential amplifier. Unfortunately, the cascomp circuit is effective only at relatively low frequencies well below the bandwidths of most differential amplifiers because among other things, the accuracy of the base-to-emitter voltage analog (i.e., the base-to-emitter voltages of Q.sub.3 and Q.sub.4) by which the actual nonlinearity is sensed decreases with increasing frequency.
It is, therefore, an object of the present invention to provide a differential amplifier having gain characteristics of improved linearity over a wider frequency range.
It is another object of the present invention to provide a differential emitter feedback amplifier system which allows accurate compensation for non-linearities in the gain of a differential amplifier.
It is a further object of the present invention to provide a differential amplifier system whose components can be readily implemented as integrated circuits.